This invention relates to electro-optical light scattering devices and particularly to such devices wherein the active medium includes a liquid crystal composition.
Thermotropic nematic liquid crystals are liquids with elongated polar molecules. One type of such crystals normally align themselves in an electric field so that their nearest neighbors are parallel to each other. This alignment causes a change in the light scattering properties of these materials. In another type of nematic liquid crystal, turbulence is caused when placed between two transparent electrodes and an electric current is sent through them disturbing the ordered state and causing the scattering of light directed onto the crystals. When this latter type of nematic liquid crystal is combined with a cholesteric material a storage effect is obtained.
The effective operation of liquid crystals in electro-optical light scattering devices is limited to a temperature range dependent upon the liquid crystal material used. This temperature range generally lies between the transition temperatures at which the liquid crystal material converts either to a solid or to an isotropic liquid.
A more detailed description of liquid crystal devices can be found with reference to an article by G. H. Heilmeier appearing in Scientific American, Volume 222, pages 100-106, April, 1970.
The ultimate commercial use of liquid crystals in many applications, such as windows, mirrors and in aircraft displays depends greatly upon the operable temperature range of the liquid crystal composition. For example, a window which can be converted from transparent to translucent may be subjected to winter temperatures of below freezing and to summer temperatures of over 100.degree. F. Consequently, the transition temperatures of the liquid crystal composition, that is, the temperatures at which the composition either crystallizes or becomes an isotropic fluid, should be below the freezing point of water and above 100.degree. F. Compositions of the present invention accomplishes this goal.